In photomechanical processes, an image formation system providing superhigh contrast photographic characteristics, especially, a gamma (.gamma.) of 10 or more, is required for satisfactory reproduction of continuous tone or line images. For this purpose, a hydroquinone developer (infectious developer) having an extremely low effective concentration of a sulfite ion, usually 0.1 mol/liter or less, has generally been used. However, such a developer is very unstable because of its low sulfite ion concentration and cannot withstand preservation for long periods of time, e.g., exceeding 3 days.
U.S. Pat. No. 2,419,975 discloses that addition of a certain hydrazine compound to a silver halide emulsion provides high contrast negative images, but superhigh contrast negative images having a gamma of 10 or more cannot be obtained with the hydrazine compounds specifically recited in this patent unless a developer having a pH value as high as 12.8 is used. Such a strongly alkaline developer having a pH in the vicinity of 13 is susceptible to air oxidation, and, therefore, cannot withstand long term preservation or use. Although preservability of the developer may be improved by increasing the sulfite ion content, a large amount of a sulfite would be necessary for sufficient improvement in stability of such a high pH developer, which leads to serious problems of not only contamination of a processing solution but hindrance to high contrast of images.
It has, therefore, been keenly desired to provide an image formation system which ensures high contrast of images and satisfactory preservability of a processing solution without being accompanied by the above described disadvantages.
One type of image formation system proposed comprises processing a surface latent image type silver halide photographic material to which a specific acylhydrazine compound that is not disclosed in the aforesaid U.S. Pat. 2,419,975 has been added with a stable developer of pH of from 11.0 to 12.3, to thereby obtain a superhigh contrast negative image, as described in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,221,857, 4,243,739, etc.
However, the above described image formation system involves a problem in that a superhigh contrast image can hardly be obtained when applied to the so-called rapid access processing enjoying an increasing demand, that is, an extremely rapid photographic processing requiring a total processing time of only from 90 to 120 seconds from the start of processing through obtaining a dried film with a time assigned to development being from 15 to 60 seconds.
On the other hand, a contrast silver halide emulsion formed in the presence of a rhodium salt has been proposed in, e.g., British Pat. No. 775,197, U.S. Pat. No. 3,531,289, etc., but the increase of contrast reached by the addition of a rhodium salt is relatively small. For example, Example 1 of U.S. Pat. No. 3,531,289 shows an increase in contrast of only from 2.60 to 3.20. In addition, use of a further increased amount of rhodium results in a reduction of the degree of blackening, i.e., maximum density. Therefore, such a silver halide emulsion fails to provide a superhigh contrast image needed for silver halide photographic materials for photomechanical processes.